Hydrologic Effects of Climate Change in the Sierra-Nevada Knowles, Noah (1); Dettinger, Michael (2); and Cayan, Daniel (2) (1) USGS, Menlo Park, CA (2) USGS; Scripps Institution of Oceanography, La Jolla, CA 1. Observed hydrologic changes a. Snowpack b. Runoff timing c. Snowfall versus Rain 2. Projected hydrologic changes a. Snowpack, snow-covered area b. Runoff timing, flow peaks c. Interannual variability d. Soil moisture Funded by the USGS NRC and Hydroclimatology Programs, NOAA OGP, DOE, the California Energy Commission’s Public Interest Energy Research Program (PIER) through the California Climate Change Center at Scripps Institution of Oceanography, and by the CALFED Science Program through the USGS CASCaDE Project.
Transcript
Hydrologic Effects of Climate Change in the Sierra-Nevada
Knowles, Noah (1); Dettinger, Michael (2); and Cayan, Daniel (2)
(1) USGS, Menlo Park, CA
(2) USGS; Scripps Institution of Oceanography, La Jolla, CA
1. Observed hydrologic changes
a. Snowpack
b. Runoff timing
c. Snowfall versus Rain
2. Projected hydrologic changes
a. Snowpack, snow-covered area
b. Runoff timing, flow peaks
c. Interannual variability
d. Soil moisture
Funded by the USGS NRC and Hydroclimatology Programs, NOAA OGP, DOE, the California Energy
Commission’s Public Interest Energy Research Program (PIER) through the California Climate Change Center at
Scripps Institution of Oceanography, and by the CALFED Science Program through the USGS CASCaDE Project.
1. Observed Hydrologic Changes
Mote et al., BAMS 2005
Snowpacks across the western U.S. and Canada have been declining.
TRENDS (1950-97) in
April 1 snow water content at
western snow measurement sites
Stewart et al., 2005
Spring-pulse dates
Centers of Mass
Also, western streamflow has been
arriving earlier in the year
in recent decades.
Also: Dettinger and Cayan, 1995; Cayan et al., 2001
Trends in ratio of winter (Nov-Mar) snowfall water equivalent (SFE) to total winter
precipitation (rain plus snow) for the period WY1949-2004. Circles represent significant
(p<0.05) trends, squares represent less significant trends.
MORE RAIN, LESS SNOW
LESS RAIN, MORE SNOW
Trends in Precipitation Form (1949-2004)
A west-wide shift from snowfall to rainfall has taken place
Knowles, N., M.D. Dettinger, and D.R. Cayan 2006. Trends in Snowfall versus Rainfall in the Western United States. Journal of Climate, 19(18), 4545–4559.
Temperature Trends
Temperature changes have driven these trends
Trends in Precipitation Form
If the warming continues, where should we expect the largest changes in warm-storm flooding to be manifested?
“Warm-storm flooding”
Less change More change
Computed from UW’s VIC model daily INPUTS (ala Bales et al, 2006)
How many
historical storms
> 1"/day were
just below
freezing?
10-yr avg
Linear trend
By comparing Western U.S. temperature, snowpack, and streamflow
timing from observations and from “naturally” and “anthropogenically”
forced GCMs, Barnett et al (Science, 2008) showed:
• Trends in all quantities are detectable (beyond natural variability).
• ~60% of 1949-1999 “trends” in these quantities are attributable to
More than half of the regional trend is anthropogenic.
Barnett, T. P., D. W. Pierce, H. G. Hidalgo, C. Bonfils, B. D. Santer, T.Das, G. Bala, A. W. Wood, T. Nozawa, A. A. Mirin, D. R. Cayan, and M. D.Dettinger, 2008: Human-induced changes in the hydrology of the westernUnited States. Science, 319, 1080-1083.
2. Projected Hydrologic Changes
…and most climate models
yield a fairly narrow range of
precipitation changes in
California.
Dettinger, 2005
19 of 23 in this range
cm
/month
cm
/month
20 of 23 in this range
Under projected greenhouse
forcings, all climate models
yield warmer futures for
California…
Pb: Not so much warmer
No ppt change
Pa: Medium warmer
Small ppt decrease
Gb: Medium warmer
Drier
Ga: Much warmer
Drier
Cayan et al., 2006, Report to
Governor
From amongst the most recent ensemble of climate-change
projections, we have chosen four scenarios to consider:
Modeling Sequence
Watershed Model
GFDL-A2 GFDL-B1
PCM-A2 PCM-B1
Avg 2070-2099 Snow Water Equivalent as percentage of 2001-2030 avg
Peak Snowpack as a Fraction of Total Annual Runoff
Northern Southern
GFDL-A2 -95% -58%
GFDL-B1 -80% -50%
PCM-A2 -74% -31%
PCM-B1 -61% -33%
GFDL-A2
Annual Maximum Snow-Covered Area
Northern Southern
GFDL-A2 -52% -23%
GFDL-B1 -46% -19%
PCM-A2 -37% -12%
PCM-B1 -26% -11%
GFDL-A2
post-April fraction of annual runoff
Northern Southern
GFDL-A2 -21% -34%
GFDL-B1 -08% -19%
PCM-A2 -23% -18%
PCM-B1 -10% -14%
GFDL-A2
Reduces snowpack leads to earlier runoff, resulting in more water being
managed as a hazard.
water
managed
as hazard
water
managed as
resource
water
managed
as hazard
water
managed as
resource
GFDL-A2 GFDL-B1 PCM-A2 PCM-B1
Changes in peak flows driven by…?
Changes in peak precipitation events
GFDL-A2 GFDL-B1 PCM-A2 PCM-B1
NOT A PREDICTION
Changes in mean annual cycles for GFDL-A2:
2070-2099 (red) vs. 2001-2030 (blue)
Reduced Soil Moisture, esp in Spring-Summer in S. Sierra
Due to reduced snowmelt and warmer air temps, stream temps also rise.
